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400                              CHEMICAL ENGINEERING
without at the same time considering purification, especially since it is frequently impossible to secure crystals of a reasonable degree of purity, or even in some cases to get crystals of the desired product at all without resorting to some process of purification.
The word purification as used in future throughout this article should be understood to be limited to the removal at any stage of manufacture of substances other than that desired as the final product, excluding, however, the immediate solvent; and irrespective of whether the impurities are removed from the solution or whether the desired substance, or the material from which it is to be derived, is separated by some process, leaving the impurities behind. This being understood, it will readily be seen why, in considering means of crystallization, means of purification play such an important part as to really be means of crystallization almost in themselves.
The most important means of crystallization or basic processes of crystallization may be briefly stated as follows:
1.  Concentration.
2.  Concentration plus cooling.
3.  Precipitation by some reagent as a comparatively pure salt of the substance desired, which salt can be readily separated and after separation can be readily decomposed, setting free the desired product in such a condition that the precipitating agent can be quite completely removed and the solution of the pure substance crystallized by concentration or other equally simple method.
4.  Cooling.
5.  Addition of a solvent for impurities in which the desired product is not, or is not readily, soluble.
6.  Addition of some substance which precipitates the desired product in a more or less crystalline form, either as applied to original solution, to a concentrated solution, or by concentration after addition of precipitant.
These means of crystallization will be discussed at length with various modifications and amplifications.
It is necessary for the sake of clearness in describing a process to consider its application to some well-known case. Because of the fact that sucrose (cane sugar) is one of the oldest chemical compounds to be manufactured and at the same time one which requires many operations to obtain in a pure crystalline form, there has perhaps been more chemical ingenuity expended upon its manufacture than upon any other product; and there are today in actual use more processes of purification and of crystallization applied to this article of commerce than to any other. There are practically no processes of purification and crystallization in use which are not typified in some of the processes of sugar manufacture. For this reason the production of this substance will be cited in a very large percentage of the processes outlined, with discussion of similar methods as applied to entirely different cases. This is not to say that these processes originated in the sugar business, for most of them were devised for other uses and adapted to sugar making. It is only that long experience has brought about a fair degree of knowledge of the multitude of methods used, and that more cases can be covered in this direction than in any other.
Concentration.—The oldest, simplest and, combined with cooling, most used method of producing crystalline solids from solutions is by concentration of the solution. The oldest application is without doubt found in the evaporation of sea water for the production of sodium chloride (common salt). Prehistoric man must have learned this process from the accumulation of salts left by the evaporation of shallow pools of sea water above tide level filled during storms and concentrated to the point of crystallization by combined sun and wind. Except in so far as the best method for securing crystals of highest purity has been